In recent years a microporous membrane made of synthetic resin is widely used for the fine filtration of fine particles in liquid or gas because of its excellent properties for the removal of the particles. There are two types of such a membrane. One type of membrane has plural micropores independently penetrating from one surface thereof to the other surface, and the other type of the microporous membrane consists of superfine resinous fibers three-dimensionaly intertwined. The former has not been used as a filter element excepting special uses, because the porosity percent of the former is smaller, so that the pressure loss is larger. Accordingly, as a filter element the latter is often used. This kind of filter has surface filter function to capture particles larger than the pore size and internal filter function to capture particles smaller than the pore size.
In the case of gas filtration the efficiency of internal filtration is so large that even particles of 1/20 of average pore size of membrane can be captured. However, such a membrane has a following serious defect in gas filtration. That is, at the filtration of gas using the filter element equipped with a membrane filter many particles to be captured by the membrane leak into the filtered gas, when an impulsive pressure such as at the initiation of the filtration is applied to the membrane, though no problem occurs when the gas passes constantly. The reasons are considered that when the impulsive pressure is applied, the flow rate of gas in the pore rapidly increases, and the ability of the membrane to capture the particles decreases, so that particles in the inlet gas can not be captured and pass through the membrane or the particles which have been captured by the membrane are scattered again into the filtered gas (the both are referred to as "re-scatter" hereinafter).
It had been believed that the rescattering did not occur in a membrane filter. Therefore, the improvement of filter element equipped with the membrane filter had been concentrated in the improvements of life and of capturing efficiency at steady flow.
On the other hand, a multi-layer filter element is known. For example, there is disclosed in Japanese Pat. KOKAI No. 82918/1984 a multi-layer filter in which the pore size of the inside filter layer is 0.3 to 0.8 times the pore size of the outside filter layer. The filter layer of the above is a close multi-layer type, but has no space between the layers, and the filter element is restrictively used for the treatment of liquid. At least it does not suggest that the provision of a space between the filter layers is effective to prevent the rescattering of dust in gas filtration.
Japanese patent publication No. 33369/1980 discloses a multi-layer filter element packed with adsorbents between filter membranes. However, this filter element has no spaces and is for the adsorption of soluble materials in water on the adsorbents. In addition, there is no suggestion to prevent the rescattering of dust in gas.
Japanese Pat. Kokai No. 61018/1985 discloses a filter element having two kinds of porous membrane made of PTFE, which are different in the pore size and are overlapped, but it suggests neither the spaces between the membranes nor the prevention of rescattering of the dust.
Japanese Utility Model Publication No. 23614/1980 discloses an oil filter having overlapped filament layers different in density. This filter can be used as air filter, but it does not suggest the existence of spaces and their effects achieved by the existence.